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SANTA FE, N.M. (AP) — A federal oversight panel reports that Los Alamos National Laboratory workers producing a shell for a triggering device for nuclear weapons violated safety rules in August by storing too much material at one location in a facility for plutonium, a highly radioactive material.

The Defense Nuclear Facilities Safety Board memorandum said workers at the northern New Mexico site discovered the placement error made by a casting crew three days later when they moved the grapefruit-sized shell again.

The memo says workers at that point failed to follow proper reporting procedures. It does specify whether the shell itself contained plutonium.

Efforts to obtain comment from the federal agency that oversees Los Alamos weren’t immediately successful. The lab said in a statement “there was no criticality accident” and that the lab “takes criticality safety very seriously and is conducting a full fact finding.” In physics, the term criticality refers to the point at which a nuclear reaction is self-sustaining.